I found a better way (for me anyway) to do the calm tile. I added a tile number to the game mode and used this on the AI spawn from the BP_Tile.
If the tile number is zero don’t spawn any AI and if the tile number is not zero use the tile number (added to a random number) to the max spawn value. This way the game gets progressively harder with more and more AI getting spawned as you progress.
Also, you don’t have to fiddle around with manually adding tiles and the game logic will just continue to work.
Don’t forget to increment the Tile Number when you spawn the tiles 
All the best and good luck,
Justin